One time programmable (OTP) memories are a cost effective alternative to flash memory technologies and may be used in applications where the memory does not need to be erased or overwritten. OTP memories are fabricated using standard CMOS processes which do not require additional masking steps. Similar to flash memories, OTP memories require high programming voltages in order to program information into the memory. Such high programming voltages are normally generated off chip in the case of an OTP memory since these voltages far exceed the highest voltage rating of available devices in standard fine line width CMOS processes that are used to manufacture microchips containing an OTP memory.
Within flash memory technologies, charge pumps are often integrated with the memory to enable the on chip generation of required high programming voltages necessary to program the flash memory. Building high voltage charge pumps within a flash memory process is fairly straightforward. Unlike standard CMOS processes used for OTP memories, flash memory manufacturing processes include special very high voltage transistors and diodes for use in the memory design. These devices allow the flash memories to tolerate the high programming voltages required to program the memory without incurring damage. These special high voltage flash devices can thus likewise be used in the design of the charge pump itself in a straight forward way, so that the charge pump can also tolerate the high programming voltage it generates for programming without incurring damage.
OTP memories are significantly less expensive than flash memories and OTP memories may be easily substituted for flash memories in many different types of applications. In these applications where an OTP memory has been substituted for a flash memory, it would be desirable to have the ability to program the OTP memory using an on chip charge pump without running the risk of damaging the circuitry of the charge pump with the necessary high programming voltages that the charge pump must itself generate for programming. It would also be desirable to allow the alternative option of programming the OTP memory using a high voltage applied external to the charge pump via an alternative source, e.g. via a programming pin on the chip package, while concurrently protecting the charge pump from the external high voltage source and consuming very low power within the charge pump. Solutions to these problems would be of great benefit to circuit designers.